I like to think of the cards in this deck in three catgories: Dying, Not Dying, and Magus Support.
Dying
Ultimately, we're trying to get our life total low, similar to Death's Shadow builds in Modern, except we're trying to go one better and have 0 or less life. Incidental damage from Marsh Flats, Godless Shrine, and Thoughtseize is great, but it doesn't really do the trick. Our main life losers are Plunge into Darkness and Spoils of the Vault.
Plunge into Darkness is preferable if we also need it to find one of our pieces, since we can dictate how much life is spent, ensuring we have 0 life at the end. Spoils of the Vault, on the other hand, can be used to find a final piece of the puzzle, but we may not lose enough life if we reveal the named card early. If we have everything we need in hand, then either one works just as well! We can always name a card that's not in our deck for Spoils of the Vault and lose as much life as we have cards in our library.
Not Dying
We rely on Angel's Grace to keep us alive at 0 life. Phyrexian Unlife provides a little backup, but the higher mana cost makes it less helpful in most cases.
An important note for Angel's Grace is that it prevents damage from reducing our life total lower than 1. However, damage causes loss of life, but loss of life is not damage. We can pay as much life as we have (including the final point of life) to Plunge into Darkness, setting our life total to 0. Spoils of the Vault can actually reduce our life total below 0. In these instances, Angel's Grace doesn't prevent our life total from going below 1, but it does prevent us from dying, which is exactly what we want!
Magus Support
Magus of the Mirror needs some help to get into play and survive a turn. Entomb can get it from our library and into the graveyard so Reanimate or Animate Dead can get it into play. Thoughtseize is mostly to clear the way for the magus to not be killed, but it can also get it into the graveyard if it happens to be in your hand.
You may have noticed there's not much interaction, or support for the magus once it's in play. That's because we live by the magus and we die by the magus! Nevertheless, we have things like Imp's Mischief, Rebuff the Wicked and Silence in the sideboard if they're needed.
